The invention relates to a circuit for monitoring the conductive state of a superconducting magnet winding. The winding is of a type which is subdivided into two series-connected sub-windings with an intermediate tap point. The ends of the sub-windings are connected via an electrically conducting parallel branch. This consists of two series-connected sub-branches, each containing a resistor of predetermined value. A junction connects the sub-branches, and this junction, together with the tap point, form the diagonal points of a bridge circuit, to which is connected an electronic device for evaluating and processing the voltage signal to be measured between these points. Such an arrangement is known from DE-OS No. 28 39 787.
Superconducting magnet windings, particularly those with a relatively high power rating, are endangered in case of an unintended transition from the superconducting operating state to the normally conducting state, even if this transition ("quenching") occurs only in a portion of the winding. In fact, due to the low heat capacity of the superconducting winding conductors, the conductor will, after such a transition, quickly heat up due to the associated increase in resistance. The conductor's specific resistance also increases rapidly, whereby the temperature rise is further acceleraed. Stray voltages also arise, presenting a strain on the insulation of the winding.
To protect relatively large superconducting magnet windings against damage or destruction by overheating or arcing, special protective measures are necessary. These measures have consisted, for example, in subdividing the magnet winding into several partial windings which for voltage limitation are bridged with protective resistors (cf. DE-OS No. 23 01 152), semiconductor diodes (cf. DE-AS No. 16 14 964) or overvoltage arresters (cf. DE-OS No. 17 64 369).
Besides these passive protective measures, active elements (particularly so-called "quench detectors") are also required. These make it possible to rapidly disconnect the superconducting magnet winding from an external energy supply in the event of quenching, particularly during the exciting phase. Such quench detectors are generally used to monitor the voltage drop across the superconducting magnet winding, which drop not only contains the ohmic voltage drop across the winding, but also the inductive signal L.times.di/dt as well as voltage signal, smaller by several factors of ten, from the remagnetization (magnetic reversal), losses of the superconductors themselves. In the simplest case only the total voltage is compared with a given maximum value. The smallest detectable ohmic signal is on the same order of magnitude of the inductive signal.
In improved monitoring circuits, the inductive signal is taken into account; the smallest detectable ohmic signal is in this case on the same order of magnitude as the signal determined by the remagnetization or magnetic reversal, losses. Still better circuits also take this signal into account. A quench detector of this kind has been described in detail in the above mentioned DE-OS No. 28 39 787.
This invention is designed for use with a superconducting magnet winding which is subdivided into two sub-windings by a center tap. Where such sub-windings consist of two symmetrically constructed halves, monitoring can be accomplished by analog comparison of the voltages across the two winding halves, in that the two sub-windings together with at least two external resistors form a bridge.
In such winding configurations, however, the ohmic resistances of e.g. solder points in the superconducting magnet winding cannot be accounted for. This limits the resolution or sensitivity of the circuit. To minimize the ohmic resistances, it is desirable to couple the external resistors of the bridge directly to the ends of the winding, but this is not always possible for reasons of space.
One object of the present invention is to increase the sensitivity of the initially mentioned circuit with respect to the detection of changes at resistances and/or inductances.
Another object is to account for the ohmic resistances of the sub-windings of the superconducting magnet winding. Still another object is, in general, to improve on the prior art.